Soft-X-ray printing has been proposed and demonstrated as a technique for replicating sub-micron planer patterns, see Soft-X-ray Lithographic Apparatus and Process, Smith et al, U.S. Pat. No. 3,743,842, July 3, 1973. Soft-X-ray exposure masks can be made from a silicon conducting crystal, see Soft X-ray Mask Support Substrate, Spears et al, U.S. Pat. No. 3,742,230, June 26, 1973. Soft-X-ray exposure masks have been made with diffraction grating patterns consisting of 1800 A wide lines on 3600 A centers and have been successfully replicated. Thus soft-X-ray lithography has shown a resolution capability far greater than that of ordinary photolithography and comparable to the highly sophisticated scanning electron microscope techniques. The simplicity and low cost of X-ray lithography indicates that it would have a significant impact on ultra-high resolution device fabrication in the future.
In the soft-X-ray lithographic process, one replicates patterns having submicron line widths by using a source of soft-X-rays, a mask member having a soft-X-ray transmissive pattern and soft-X-ray absorber patterns of submicron thickness whose absorption of soft-X-rays produces a soft-X-ray image of the mask pattern, and a reproduction member consisting of a substrate and a soft-X-ray sensitive layer supported on said substrate, said sensitive layer being between said substrate and said mask for absorbing soft-X-rays in the pattern created by the mask. However, due to the finite size of the soft-X-ray source, there will be a penumbral blurring of the image of the pattern on the mask, the degree of said penumbra being related directly to the spacing between the pattern on the mask and the X-ray sensitive layer which records the image, i.e., the larger the distance, the larger the penumbral blurring of the image. During exposures in which this blurring must be minimized, then, one has to reduce the distance between the mask and the X-ray sensitive layer to zero, that is, to cause intimate contact.
X-ray masks, while able to be made from a variety of materials, usually consist of thin, X-ray transparent and opaque patterned membranes supported by thicker, self-supporting parts for mechanical strength. The thin X-ray patterned membrane may be silicon, aluminum, beryllium or a polymer material such as Mylar. The structure may be one continuous crystalline material or may be a thin patterned membrane attached to any mechanically suitable support.
Initial attempts to place the X-ray mask membrane in intimate contact with the X-ray sensitive layer were performed by simply applying mechanical pressure to the thick areas of the mask to press it into contact with the X-ray sensitive layer. Experimentally it was found that, while the thick areas were pressed onto the X-ray sensitive layer, the thin, flexible, X-ray transparent membrane would in most cases not come into intimate contact with the X-ray sensitive layer. This is primarily due to two reasons. One is the presence of dust particles which may be in the space between the mask and the sample and will prevent the mask from lying flat in intimate contact with the X-ray sensitive layer. The second reason is that the surface of the X-ray mask and the surface of the substrate supporting the X-ray sensitive layer may not themselves be flat, and even in the complete absence of any dust particles, the mask will rest on the high points of the X-ray sensitive layer with a finite gap existing elsewhere between the mask and other portions of the X-ray sensitive layer.
The fundamental problem is that the mechanical pressure is applied at discrete points on the thick structure of the mask while the pressure actually exerted on the thin, flexible X-ray transparent membrane itself is that produced only through its tensional attachment to the thick structure.